Is there a way to declare a custom SQL index?

I'm trying to create an index on the lowercase value of a column using Postgres.

I tried

unique().on(sql`lower(${table.username}::text)`)

unique().on(sql`lower(${table.username}::text)`)

unique().on(sql`lower(${table.username}::text)`)

unique().on(sql`lower(${table.username}::text)`)

but "on" only accepts columns